This is all very well, but it does beg the `free will' question. It's a deterministic explanation, valid for idealised physical systems like billiard balls. Now, it is possible that the human mind is actually a deterministic system (ignoring quantum effects to keep the discussion within bounds). What we like to think of as making a free choice may actually be what it feels like when a deterministic brain works its way towards the only decision that it can actually reach. Free will may be the 'quale' of decision-making - the vivid feeling we get, like the vivid sense of colour we get when we look at a red flower.' Physics does not yet explain. how these feelings arise. So it is usual to dismiss effects of free will when discussing possible temporal paradoxes.

This sounds reasonable, but there's a catch. The whole discussion of time machines, in physics terms, is about the possibility of people constructing the various warped spacetimes that are involved. `Get a black hole, join it to a white one ...' Specifically, it is about people choosing or deciding to construct such a device. In a deterministic world, either they are bound to construct it from the beginning, in which case `construct' isn't a very appropriate word, or the thing just puts itself together, and you find out what sort of

[1] See Ian Stewart, Jack Cohen, Figments of Reality: the origins of the curious mind (Cambridge University Press, universe you are in. It's just like Godel's rotating universe: either you're in it, or you're not, and you don't get to change anything. You can't bring a time machine into being unless it was already implicit in the unfolding of that universe anyway.

The standard physics viewpoint really only makes sense in a world where people have free will and can choose to build, or not to build, as they see fit. So physics, not for the first time, has adopted inconsistent viewpoints for different aspects of the same question, and has got its philosophical knickers in a twist as a result.

For all the clever theorising, the dreadful truth is that we do not yet have the foggiest idea how to make a practical time machine. The clumsy and energy-wasteful devices of real physics are a pale shadow of the elegant machine of Wells's Time Traveller, whose prototype was described as `a glittering metallic framework, scarcely larger than a small clock, very delicately made. There was ivory in it, and some transparent crystalline substance.'

There's still some R&D needed.

Probably this is a Good Thing.

AVOIDING MADEIRA

THE JOINER WAS AMAZED, As he told his mates in the pub after work

- so I was just finishing, and this feller comes down the ladder and says beggin' your pardon, sir, but I'd just like to check that bulkhead, please. Nothing wrong with it, says I, it's as sound as a bell. Then he says, right, right, of course, but I've just got to check something. He pulls this piece of paper out of his pocket and reads it careful, and says he's got to check that the new timber hasn't got a rare tropical worm that'll leave it looking like good wood but weaken it so much that the ship will take in too much water and will have to put in to Madeira for repairs, or something, possibly. I'll soon see about that, says I and whacks it with my hammer and, blow me, it cracks in half. I'd have sworn it was prime timber, too. Little worms everywhere!'

`Funny you should say that,' said the man opposite. `One of 'em came up when I was working and asked if he could look at the copper nails I was usin'. Well, he takes out a knife, scrapes away at one, it's a bit of rubbish iron under a skin 'o copper! Had to do half a day's work again! Beats me how he knew. Tom said the chandler swore they were all copper when he supplied 'em.'

`Hah,' said a third man, `one came up to me and said what would I do if a giant squid pulled the ship under. I told him I'd do nothing, being as I live in Portsmouth.' He drained his mug. `Damned thorough, these inspectors.'

`Yeah,' said the first man, reflectively. `They think of everything. ..'

`A goose is an inconvenient bird, I've always thought,' said Mustrum Ridcully, carving it. Just a bit too much for one but not quite enough for two.' He extended a fork. `Anyone else want some? Rincewind, just get the man to send up some more oysters, will you? What do you say, gentlemen? Another six dozen? Let's push the boat out, eh? Hahah ...'

The wizards had taken rooms at an inn, and the owner, watching the bustling staff down in his kitchen, was already thinking happily of an early retirement.

Money had not been a problem. Hex had merely teleported some from a distant bank. The wizards had debated the moral implications of this for some time, with their mouths full, but had come down in favour of the idea. They were, after all, Doing The Right Thing.

Only Ponder wasn't eating much. He nibbled a biscuit and updated his notes, before announcing: `We have covered everything, Archchancellor. The nails, the leaking water barrels, the defective compass, the bad meat ... there were nine reasons why the Beagle would have called in at the island of Madeira. Hex believes the giant squid may be a red herring. As for the nine ... yes, I think we have assured that they will no longer occur.'

`Remind me why that's important, will you?' said the Dean. `And pass the wine, Mustrum.'

`Without this intervention it's more than likely that Darwin will leave the ship at Madeira, should the Beagle call there,' said Ponder. `He will be terribly seasick on the voyage.'

`Madeira being - ?' said the Dean.

'One of a group of islands on the way, Dean. After that it's a long haul down to the South Atlantic, round the bottom of South America with a few stops, and straight up to the Galapagos Islands.'

`Down, bottom, up,' muttered the Dean. `How can anyone get the hang of globular navigation?'

`The phenomenon we call The Love Of Iron, sir,' said Ponder, smoothly. 'We only find it in rare metals that drop from the sky, but it's very common here. Iron here tries to point north.'

Silence fell around the table.

`North? Is that the bit at the top?' said Ridcully.

`Conventionally, sir, yes,' said Ponder, and rather foolishly added,

`but on a globe it doesn't really matter, of course.'

`Ye gods,' muttered the Dean, putting his hand over his eyes. `How does the iron know which way to point?' Ridcully persisted.

`Metal can't think.'

`It's a bit like ... like peas turning to follow the Sun, sir,' hazarded Ponder, not sure if they actually did; perhaps it was pea farmers.

`Yes, but peas are living things,' said Ridcully. `They ... know about the Sun, right?'

`Peas aren't exactly renowned for their brains, Archchancellor,' said the Chair of Indefinite Studies, `hence the term pea-brained.'

`But a pea must be a bloody genius compared to a lump of iron, yes?' said Ridcully.

Ponder knew he had to put a stop to this. The wizards were still determined to apply common sense to Roundworld, and that would get them nowhere.

`It's a force that can occur on globe-shaped worlds,' he said. `It's caused by the molten iron core spinning, and helps prevent life on the surface being fried by the Sun.'

`Sounds like Deitium in disguise, doesn't it,' said Ridcully. `Planet gets this big magical umbrella so that life can survive? Shows forethought.' `It doesn't work quite like that, Archchancellor,' said Ponder. `Life evolved because conditions allowed it to do so.'

`Ah, but if conditions hadn't been right, there wouldn't have been any life,' said Ridcully. `Therefore the whole exercise would have been pointless.'

`Not really, sir. There wouldn't have been anyone to point out the pointlessness of it,' said Ponder. `I was about to add that some birds, like pigeons, use The Love Of iron to help them navigate long distances. They have tiny things called "magnets" in their head, says Hex. They're ... little bits of iron that know where the North Pole is